1. Field of the Invention
The present invention relates to a sheet manufacturing method and a manufacturing apparatus of manufacturing a sheet (a plate-like base) from a melt containing a metal material or a semiconductor material. Particularly, the present invention relates to a technique for inexpensively manufacturing solar cells, where a silicon sheet for a solar cell is directly manufactured from a silicon melt. More particularly, the present invention relates to a silicon sheet manufacturing apparatus and method enabling manufacture of a silicon sheet, which has protrusions or curved portions at least on the melt side when viewed in section.
2. Description of the Background Art
Examples of conventional methods of manufacturing polycrystalline silicon wafers for use in solar cells include a method of casting to form a polycrystalline body of e.g., silicon, disclosed in Japanese Patent Laying-Open No. 6-64913. In this method, a high-purity silicon material containing a dopant of phosphorus, boron, or the like, is heated to melt in a crucible placed in an inert ambience. Then, a silicon melt is poured into a casting mold for gradually cooling, so as to provide a polycrystalline ingot. Accordingly, for manufacturing polycrystalline silicon wafers for use in solar cells from thus obtained polycrystalline ingot, the ingot is to be sliced by a wire saw or inner diameter blade.
A method of continuously casting a silicon plate, disclosed in Japanese Patent Laying-Open No. 7-256624, involves manufacture of a silicon sheet without slicing. In this method, a silicon melt is poured into a horizontal heat casting mold and a dummy graphite plate is horizontally inserted such that its leading end is dipped into the silicon melt under a control plate. When the silicon adheres to the leading end of the graphite plate, the silicon plate is horizontally pulled out with use of a roller. Coolant gas supplied from gas blow-off pipe of a cooling apparatus provides for continuous formation of the silicon plate.
Another method of manufacturing a silicon sheet uses a manufacturing apparatus for a silicon ribbon disclosed in Japanese Patent Laying-Open No. 10-29895. The manufacturing apparatus for a silicon ribbon generally has a portion of heating to melt silicon and a cooling roller of a heat-resistant material. The cooling roller, with one end of carbon net wound thereon, is directly dipped into a silicon melt for forming a silicon ribbon on the surface of the cooling roller. The carbon net wound on the cooling roller is pulled with rotation of the cooling roller for rolling out thus formed silicon ribbon. As such, the manufacturing apparatus allows the silicon ribbon, formed from the silicon that first adhered to the carbon net, to be continuously rolled out.
However, the above described conventional methods or apparatuses of manufacturing a silicon plate or a silicon sheet suffer from the following problems. The method of casting a crystalline body of e.g. silicon disclosed in the aforementioned laid-open application No. 6-64913 requires slicing of the polycrystalline ingot, whereby a slicing loss is caused corresponding to a thickness of the wire saw or inner diameter blade. This results in yield decrease and it becomes difficult to provide wafers at low cost.
The method of continuously casting the silicon plate disclosed in the aforementioned laid-open application No. 7-256624 controls the thickness of the silicon plate by pulling out the silicon plate under the thickness control plate. In this case, it is difficult to control a thickness of 600 μm or smaller as is employed for solar cells.
In the method of manufacturing the silicon ribbon disclosed in the aforementioned laid-open application No. 10-29895, the silicon ribbon formed from the silicon that first adhered to the carbon net is continuously pulled and rolled out with rotation of the cooling roller. However, the silicon ribbon is somewhat fragile due to reaction of the carbon net and the silicon. If the formed silicon is extremely thin, the silicon ribbon may break to fall during pulling operation. In this situation, the operation must be stopped and productivity decreases.
Further, a mechanism is provided which pressurizes to apply the silicon melt onto the peripheral surface of the cooling roller by jet pressure. Since the pressure is exerted by agitation of the silicon melt, unwanted pressure may be exerted to the formed silicon, thereby causing defects.
A growth rate of silicon is determined by a number of factors including a temperature of a heater for maintaining the silicon in a molten state, dipping depth, type and flow rate of coolant gas circulating in the cooling roller, rotation speed of the cooling roller and the like. Thus, it is technically difficult to stably pull out the silicon ribbon while controlling the growth rate.
Further, a wedge is provided for removing any silicon residue left on the surface of the cooling roller. Such a wedge is brought into direct contact with the surface of the cooling roller where silicon grows, thereby disadvantageously scratching the surface of the cooling roller or striping a remover applied thereon. Consequently, evenness of the silicon ribbon is impaired.
An inexpensive solar cell requires a base which has excellent evenness and well-controlled thickness and which saves a slicing loss. In any of these conventional cases, it is difficult to provide a thin base with excellent evenness manufactured by mass production at low cost.